President Donald Trump has said the US will take very strong action against Iran if it executes protesters, as rights groups say more than 2,400 anti-government demonstrators have been killed in a violent crackdown by Iranian authorities.
Relatives of 26-year-old Erfan Soltani, who was detained last week, have told BBC Persian that he is due to be executed on Wednesday.
A representative from the Hengaw Organization for Human Rights also told the BBC that they had never witnessed a case move so quickly.
Speaking to the BBC's US partner CBS News, Trump said of potential executions: If they hang them, you're going to see some things... We will take very strong action if they do such a thing.
Soltani's relative told BBC Persian that an Iranian court had issued a death sentence in an extremely rapid process, within just two days.
Awyar Shekhi, a representative of Hengaw, said that the case demonstrated that the Iranian government is using every tactic they know to suppress people and spread fear.
An Iranian official told Reuters that 2,000 people had been killed, but that terrorists were to blame.
Trump earlier said he planned to attend a meeting at the White House on Tuesday night to discuss the situation in Iran, pledging to get accurate numbers on the death toll from the protests.
The US-based Human Rights Activists News Agency (HRANA) said that it had so far confirmed the killing of 2,403 protesters, as well as 12 children, despite an internet blackout. Nearly 150 people affiliated with the government have also been killed, the group said.
The killing looks like it's significant, but we don't know yet for certain, Trump told reporters while returning to the White House.
Once he has the numbers, he said, we'll act accordingly.
Earlier on Tuesday, Trump wrote on his Truth Social platform that Iranian authorities would pay a big price for the killings, and urged people to keep protesting.
He added, I have cancelled all meetings with Iranian Officials until the senseless killing of protesters STOPS. HELP IS ON ITS WAY. MIGA!!!
Trump has been weighing military and other options in response to the crackdown, having already announced 25% tariffs on any country trading with Iran.
Iran's government has responded by accusing the US of seeking to manufacture a pretext for military intervention, warning that this playbook has failed before.
The protests, which have reportedly spread to 180 cities and towns in all 31 provinces, were sparked by anger over the collapse of the Iranian currency and soaring cost of living.
They quickly widened into demands for political change and became one of the most serious challenges to the clerical establishment since the 1979 Islamic revolution.
Demonstrations escalated significantly last Thursday and were met with deadly force by authorities, masked by a near total shutdown of the internet and communication services.
More than 18,434 protesters have been arrested during the unrest, according to HRANA.
It is difficult to gauge the true scale of bloodshed because, like other international news organizations, the BBC is not able to report from inside the country.
However, videos posted online on Sunday showed people searching for the bodies of their loved ones at the Kahrizak Forensic Centre in Tehran. The BBC counted at least 180 shrouded bodies and body bags in the footage.
Hospitals in the capital have also reportedly been overwhelmed by the number of casualties.
Prof Shahram Kordasti, an Iranian oncologist based in London, told the BBC's Newsday programme on Tuesday that the last message he had received from a colleague in Tehran said: In most hospitals, it's like a warzone. We are short of supplies, short of blood.
Other doctors at two to three hospitals had also said they had treated hundreds of injured or dead people, he added.
New videos of protests in recent days have also emerged, with BBC Persian verifying those filmed in the central city of Arak and the western cities of Tabriz, Urmia and Khorramabad.
The protesters chant slogans Death to the dictator - a reference to Ayatollah Khamenei - and Reza Shah, may your soul rest in peace - referring to the late monarch Mohammad Reza Pahlavi.
